This invention relates to the field of dielectric optical waveguides and optical telecommunications.
Optical waveguides guide optical signals to propagate along a preferred path or paths. Accordingly, they can be used to carry optical signal information between different locations and thus they form the basis of optical telecommunication networks. The most prevalent type of optical waveguide is an optical fiber based on index guiding. Such fibers include a core region extending along a waveguide axis and a cladding region surrounding the core about the waveguide axis and having a refractive index less than that of the core region. Because of the index-contrast, optical rays propagating substantially along the waveguide axis in the higher-index core can undergo total internal reflection (TIR) from the core-cladding interface. As a result, the optical fiber guides one or more modes of electromagnetic (EM) radiation to propagate in the core along the waveguide axis. The number of such guided modes increases with core diameter. Notably, the index-guiding mechanism precludes the presence of any cladding modes lying below the lowest-frequency guided mode. Almost all index-guided optical fibers in use commercially are silica-based in which one or both of the core and cladding are doped with impurities to produce the index contrast and generate the core-cladding interface. For example, commonly used silica optical fibers have indices of about 1.45 and index contrasts of up to about 2-3% for wavelengths in the range of 1.5 microns.
Signals traveling down an optical fiber slowly attenuate, necessitating periodic amplification and/or regeneration, typically every 50-100 km. Such amplifiers are costly, and are especially inconvenient in submarine cables where space, power sources, and maintenance are problematic. Losses for silica-based optical fibers have been driven down to about 0.2 dB/km, at which point they become limited by the Rayleigh scattering processes. Rayleigh scattering results from microscopic interactions of the light with the medium at a molecular scale and is proportional to xcfx894xcfx81, where 107  is the light frequency and xcfx81 is the material density, along with some other constants of the material.
In addition to loss, signals propagating along an optical fiber may also undergo nonlinear interactions. In an ideal linear material, light does not interact with itselfxe2x80x94this is what allows a fiber to carry multiple communications channels simultaneously in separate wavelengths (wavelength-division multiplexing, or WDM), without interactions or crosstalk. Any real optical medium (even vacuum), however, possesses some nonlinear properties. Although the nonlinearities of silica and other common materials are weak, they become significant when light is propagated over long distances (hundreds or thousands of kilometers) or with high powers. Such nonlinear properties have many undesirable effects including: self/cross phase modulation (SPM/XPM), which can cause increased pulse broadening and limit bitrates; and a four-wave mixing (FWM) and stimulated Raman/Brillouin scattering (SRS/SBS), which induce crosstalk between different wavelength channels and can limit the number of achievable channels for WDM. Such nonlinearities are a physical property of the material in the waveguide and typically scale with the density of the waveguide core.
Typically, optical fibers used for long-distance communications have a core small enough to support only one fundamental mode in a desired frequency range, and therefore called xe2x80x9csingle-modexe2x80x9d fibers. Single mode operation is necessary to limit signal degradation caused by modal dispersion, which occurs when a signal can couple to multiple guided modes having different speeds. Nonetheless, the name xe2x80x9csingle-modexe2x80x9d fiber is something of a misnomer. Actually, single-mode fibers support two optical modes, consisting of the two orthogonal polarizations of light in the fiber. The existence and similarity of these two modes is the source of a problem known as polarization-mode dispersion (PMD). An ideal fiber would possess perfect rotational symmetry about its axis, in which case the two modes would behave identically (they are xe2x80x9cdegeneratexe2x80x9d) and cause no difficulties. In practice, however, real fibers have some acircularity when they are manufactured, and in addition there are environmental stresses that break the symmetry. This has two effects, both of which occur in a random and unpredictable fashion along the fiber: first, the polarization of light rotates as it propagates down the fiber; and second, the two polarizations travel at different speeds. Thus, any transmitted signal will consist of randomly varying polarizations which travel at randomly varying speeds, resulting in PMD: pulses spread out over time, and will eventually overlap unless bit rate and/or distance is limited. There are also other deleterious effects, such as polarization-dependent loss. Although there are other guided modes that have full circular symmetry, and thus are truly xe2x80x9csingletxe2x80x9d modes, such modes are not the fundamental modes and are only possible with a core large enough to support multiple modes. In conventional optical fibers, however, the PMD effects associated with the fundamental mode of a small core supporting only a xe2x80x9csingle-modexe2x80x9d are far preferable to the effects of modal dispersion in a larger core multi-mode fiber.
Another problem with directing optical signals along an optical waveguide is the presence of chromatic or group-velocity dispersion in that waveguide. Such dispersion is a measure of the degree to which different frequencies of the guided radiation propagate at different speeds (i.e., group velocities) along the waveguide axis. Because any optical pulse includes a range of frequencies, dispersion causes an optical pulse to spread in time as its different frequency components travel at different speeds. With such spreading, neighboring pulses or xe2x80x9cbitsxe2x80x9d in an optical signal may begin to overlap and thereby degrade signal detection. Thus, absent compensation, dispersion over an optical transmission length places an upper limit on the bit-rate or bandwidth of an optical signal.
Chromatic dispersion includes two contributions: material dispersion and waveguide dispersion. Material dispersion comes from the frequency-dependence of the refractive index of the material constituents of the optical waveguide. Waveguide dispersion comes from frequency-dependent changes in the spatial distribution of a guided mode. As the spatial distribution of a guided modes changes, it sample different regions of the waveguide, and therefore xe2x80x9cseesxe2x80x9d a change in the average index of the waveguide that effectively changes its group velocity. In conventional silica optical fibers, material dispersion and waveguide dispersion cancel each other out at approximately 1310 nm producing a point of zero dispersion. Silica optical fibers have also been modified to move the zero dispersion point to around 1550 nm, which corresponds to a minimum in material absorption for silica.
Unfortunately, while operating at zero dispersion minimizes pulse spreading, it also enhances nonlinear interactions in the optical fiber such as four wave mixing (FWM) because different frequencies remain phase-matched over large distances. This is particularly problematic in wavelength-division multiplexing (WDM) systems where multiple signals are carried at different wavelengths in a common optical fiber. In such WDM systems, FWM introduces cross talk between the different wavelength channels as described above. To address this problem, WDM systems transmit signals through optical fibers that introduce a sufficient dispersion to minimize cross-phase modulation, and thereafter transmits the signals through a xe2x80x9cdispersion compensating fiberxe2x80x9d (DCF), to cancel the original dispersion and minimize pulse spreading in the compensated signal. Unfortunately, aggregate interactions between the dispersion and other nonlinear processes such as self-phase modulation can complicate dispersion compensation.
Another type of waveguide fiber, one that is not based on TIR index-guiding, is a Bragg fiber, which includes multiple dielectric layers surrounding a core about a waveguide axis. The multiple layers form a cylindrical mirror that confines light to the core over a range of frequencies. The multiple layers form what is known as a photonic crystal, and the Bragg fiber is an example of a photonic crystal fiber. Some researchers have commented that Bragg fibers are not feasible for long distance optical transmission (see N. J. Doran and K. J. Blow, J. of Lightwave Tech., LT-1:588, 1983).
The invention features a photonic crystal fiber having properties particularly suitable for use in optical transmission. In particular, the photonic crystal fibers described herein have low losses, exhibit small nonlinear effects, and can effectively operate in a non-degenerate single mode.
The inventors have recognized that designing a photonic crystal fiber (such as a Bragg fiber) with a large core radius (e.g., larger than about twice the wavelength of the guided radiation) leads to many desirable properties. For example, the inventors have determined that the fraction of energy outside of the core for a guided mode in a photonic crystal fiber scales inversely with the cube of the core radius. Accordingly, radiation and dissipation losses associated with the dielectric confinement layers can be made very small by increasing the core radius. Moreover, because the confinement mechanism is not based on total internal reflection (TIR), the core material is not limited to a material having a relatively high index. Thus, the core material can be selected to minimize losses and nonlinearities. For example, the fiber may have a hollow core. Furthermore, the inventors have discovered that confinement in the core is further improved by selecting materials for the layers (or regions) outside the core to have a large contrast in refractive index. Such contrasts are possible because the large core radius makes dissipation by the outside layers (or regions) less of an issue and thus the constituent materials of the outside layers (or regions) can be selected more on the basis of providing the desired index contrast, than on absorption losses.
The inventors have further recognized that although the large core leads to multiple guided modes, the multiple modes have attenuation losses that differ significantly from one another. The differential losses among the multiple modes (i.e., modal filtering) rapidly lead to single-mode operation for modest transmission lengths, and thus modal dispersion is avoided. Moreover, the lowest-loss mode can be selected to be non-degenerate, which eliminates effects such as polarization-mode dispersion. Nonetheless, the inventors have also recognized that there is an upper limit on the core size. When the core size is too large (e.g., larger than about forty times the wavelength of the guided radiation), the modes become closely spaced, and thus perturbations more easily cause coupling between different modes. Also, the inventors have recognized that in Bragg fibers, the TE01 mode has a node near the core/cladding interface, which leads to a reduction in losses and nonlinear effects.
We will now summarize different aspects, features, and advantages of the invention.
In general, in one aspect, the invention features an optical waveguide including: (i) a dielectric core region extending along a waveguide axis; and (ii) a dielectric confinement region surrounding the core about the waveguide axis, the confinement region including a photonic crystal structure having a photonic band gap, wherein during operation the confinement region guides EM radiation in at least a first range of frequencies to propagate along the waveguide axis. The core has an average refractive index smaller than about 1.3 for a frequency in the first range of frequencies, the core has a diameter in a range between about 4 xcex and 80 xcex, wherein xcex is a wavelength corresponding to a central frequency in the first frequency range, and the dielectric confinement region extends transversely from the core for at least a distance of about 6 xcex.
In general, in another aspect, the invention features an optical waveguide including: (i) a dielectric core region extending along a waveguide axis; and (ii) a dielectric confinement region surrounding the core about the waveguide axis. The confinement region has an average index greater than that of the core, and during operation the confinement region guides EM radiation in at least a first range of frequencies to propagate along the waveguide axis. The core has an average refractive index smaller than about 1.3 for a frequency in the first range of frequencies, the core has a diameter in a range between about 4 xcex and 80 xcex, wherein xcex is a wavelength corresponding to a central frequency in the first frequency range, and the dielectric confinement region extends transversely from the core for at least a distance of about 6 xcex.
In general, in another aspect, the invention features an optical waveguide including: (i) a dielectric core region extending along a waveguide axis; and (ii) a dielectric confinement region surrounding the core about the waveguide axis. The confinement region includes alternating layers of at least two dielectric two materials surrounding the core about the waveguide axis, the two dielectric materials having refractive indices that differ by at least 10%, and wherein during operation the confinement region guides EM radiation in at least a first range of frequencies to propagate along the waveguide axis. The core has an average refractive index smaller than about 1.3 for a frequency in the first range of frequencies, the core a diameter in a range between about 4 xcex and 80 xcex, wherein xcex is a wavelength corresponding to a central frequency in the first frequency range, and the dielectric confinement region extends transversely from the core for at least a distance of about 6 xcex.
In general, in another aspect, the invention features an optical waveguide including: (i) a dielectric core region extending along a waveguide axis; and (ii) a dielectric confinement region surrounding the core about the waveguide axis. The confinement region includes at least 12 pairs of alternating layers of dielectric material having different refractive indices, wherein the layers are sufficient to guide EM radiation in at least a first range of frequencies to propagate along the waveguide axis. The refractive indices of the alternating layers differ by at least 10% for a frequency in the first range of frequencies. At least some of the pairs of alternating layers have a total thickness equal to about a, and the core has a diameter in a range between about 10 a and 100 a. In some embodiments, the core diameter is in a range between 20 a and 80 a.
In another aspect, the invention features an optical waveguide including: (i) a dielectric core region extending along a waveguide axis; and (ii) a dielectric confinement region surrounding the core about the waveguide axis. The confinement region guides EM radiation in at least a first range of frequencies to propagate along the waveguide axis. The core has an average refractive index smaller than about 1.3 for a frequency in the first range of frequencies, and the core has a diameter in a range between about 5 microns and 170 microns.
In general, in another aspect, the invention features an optical waveguide including: (i) a dielectric core region extending along a waveguide axis; and (ii) a dielectric confinement region surrounding the core about the waveguide axis. The confinement region includes at least two dielectric materials forming a photonic crystal structure having a photonic band gap, the dielectric materials sufficient to guide EM radiation in at least a first range of frequencies to propagate along the waveguide axis. The refractive indices of the dielectric materials in the confinement region differ by at least 10% for a frequency in the first range of frequencies, and the core has a diameter in a range between about 5 microns and 170 microns.
Embodiments of any of the waveguides described above may include any of the following features.
The dielectric confinement region may extend transversely from the core for at least a distance of about 8 xcex, about 10 xcex, or about 12 xcex. The average refractive index of the core may smaller than about 1.3, smaller than about 1.2, or smaller than about 1.1 for a frequency in the first range of frequencies. The core may include a gas.
The diameter of the core may be in a range have a lower limit of any of 4 xcex, 6 xcex, 8 xcex, or 10 xcex and an upper limit of any of 100 xcex, 80 xcex, 60 xcex, or 40 xcex, wherein xcex is a wavelength corresponding to a central frequency in the first range of frequencies.
The diameter of the core may be in a range have a lower limit of any of 5 microns, 7 microns, 10 microns, and 12 microns and an upper limit of any of 170 microns, 120 microns, and 100 microns, and 50 microns.
The first range of frequencies may correspond to wavelengths in the range of about 1.2 microns to 1.7 microns. Alternatively, the first range of frequencies may correspond to wavelengths in the range of about 0.7 microns to 0.9 microns. The ratio of the bandwidth of the first range of frequencies and the central frequency and may be at least about 10%.
The waveguide axis may be substantially straight or it may include one or more bends. The core may have a circular cross-section, a hexagonal cross-section, or a rectangular cross-section.
The confinement region may guide at least one mode to propagate along the waveguide axis with radiative losses less than 0.1 dB/km, or even less than 0.01 dB/km for a frequency in the first range of frequencies. For example, the mode may be a TE mode (e.g., TE01). The waveguide may support a mode in which at least 99% of the average energy of the propagating EM radiation is in the core for a frequency in the first range of frequencies.
The confinement region may include at least two dielectric materials having different refractive indices. The ratio of the refractive index of the higher index dielectric material to that of the lower index dielectric material may be greater than 1.1, greater than 1.5, or greater than 2. For example, the lower-index dielectric material may include a polymer or a glass, and the higher-index dielectric material may include germanium, tellurium, or a chalcogenide glass.
The photonic bandgap may be an omnidirectional photonic bandgap. The photonic bandgap may be sufficient to cause EM radiation that is incident on the confinement region from the core in the first frequency range and with any polarization to have a reflectivity for a planar geometry that is greater than 95% for angles of incidence ranging from 0xc2x0 to at least 80xc2x0. The photonic crystal may be a a one-dimensional photonic crystal or a two-dimensional photonic crystal.
The confinement region may include alternating layers of the two dielectric material surrounding the core about the waveguide axis. For example, the refractive indices and thicknesses of at least some of the alternating dielectric layers substantially satisfy the following equality:                     d        hi                    d        lo              =                                        n            lo            2                    -          1                                                  n            hi            2                    -          1                      ,
where dhi and dlo are the thicknesses of adjacent higher-index and lower-index layers, respectively, and nhl and nlo are the refractive indices of the adjacent higher-index and lower-index layers, respectively. The confinement region may include at least 12 pairs of the alternating layers. The waveguide may support at least one mode propagating along the waveguide axis for which the confinement region includes a sufficient number of pairs of alternating layers to limit radiative losses of the mode to less than 0.1 dB/km or even less than 0.01 dB/km for a frequency in the first range of frequencies.
At least a first end of the waveguide may include a coupling segment over which the refractive index cross-section is continuously varied to alter the field profile of the working mode. Furthermore, there may be a second waveguide coupled to the first mentioned waveguide, wherein the cross-section of the second waveguide adjacent the first waveguide includes regions of doped silicon located to improve coupling of the working mode into the second waveguide. Alternatively, or in addition, the cross-section of the second waveguide adjacent the first waveguide may include a hollow ring contacting the dispersion tailoring region of the first waveguide to thereby improve coupling of the working mode into the second waveguide.
In another aspect, the invention features an optical telecommunications system including: (i) a transmitter generating an optical signal; and (ii) any of the optical waveguides described above coupled at one end to the transmitter to carry the optical signal, wherein the optical signal is at a frequency in the first frequency range. The optical waveguide may have a length greater than 30 km, greater than 200 km, or greater than 500 km.
The system may further include an optical receiver coupled to the other end of the optical waveguide to detect the optical signal, an optical amplifier coupled to the other end of the optical waveguide to amplify the optical signal, an optical regenerator coupled to the other end of the optical waveguide to regenerate the optical signal as an electrical signal, and/or a dispersion compensation module coupled to the other end of the optical waveguide to introduce dispersion to the optical signal that substantially cancels dispersion caused by the optical waveguide. The optical signal may be at a wavelength in the range of about 1.2 microns to about 1.7 microns or in the range of about 0.7 microns to about 0.9 microns. Furthermore, the transmitter may generate multiple signals at different wavelengths, and wherein the different wavelengths correspond to frequencies in the first frequency range.
In general, in another aspect, the invention features a method of designing a photonic crystal optical waveguide including a dielectric core region extending along a waveguide axis and a dielectric confinement region surrounding the core about the waveguide axis, wherein the confinement region is configured to guide EM radiation in at least a first range of frequencies to propagate along the waveguide axis and wherein the core has an average refractive index smaller than about 1.3 for a frequency in the first range of frequencies. The method includes selecting a diameter for the core based on one or more design criteria for the guided EM radiation including mode separation, group-velocity dispersion, radiative losses, absorption losses, and cladding nonlinearity suppression. For example, the diameter for the core may be based on at least two of the design criteria. In particular, an upper limit for the diameter of the core may be selected based on the mode separation, and a lower limit for the diameter may be selected based on at least one of the group-velocity dispersion, the radiative losses, the absorption losses, and the cladding nonlinearity suppression. Furthermore, the confinement region may include at least two dielectric materials having different refractive, and the method may further include selecting an index contrast for the different refractive indices based on at least one of the design criteria including the radiative losses, the absorption losses, and the cladding nonlinearity suppression.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.